Perpendicular magnetic recording medium, method for producing the same, and sputtering device

ABSTRACT

A magnetic recording medium conventionally utilizes the in-plane magnetization mode, but recently the perpendicular magnetization mode utilizing the perpendicular anisotropy of an hcp cobalt alloy layer, in which the C axis is oriented perpendicular to the layer surface, has been proposed. The known perpendicular magnetic recording medium is produced by means of RF sputtering and comprises a Permalloy layer, as layer of a low coercive-force material, between the nonmagnetic base and the hcp cobalt alloy layer. The perpendicular anisotropy attained by the present invention in very excellent and is superior to that of a perpendicular recording medium having no Permalloy layer because a Co-Ta alloy is used as the layer of a low coercive-force material.

This is a division of application Ser. No. 819,229, filed Jan. 15, 1986,now U.S. Pat. No. 4,666,788, issued May 19, 1987 which is a divisionalof U.S. Ser. No. b 466,683 filed Feb. 16, 1983, now U.S. Pat. No.4,576,700, issued Mar. 18, 1986.

The present invention relates to a perpendicular magnetic recordingmedium of a cobalt alloy which comprises mainly cobalt, and additionallychromium, and a method for producing the same. More particularly, thepresent invention relates to a perpendicular magnetic recording mediumcomprising a nonmagnetic base, a layer of a low coercive force material,and a layer of a cobalt alloy, as well as a method for producing thesame. In addition, the present invention also relates to an improvedsputtering device.

The present magnetic recording systems fundamentally use thelongitudinal (in-plane) magnetization mode, that is, a magnetizationbeing parallel to the base, to which the cobalt alloy is applied.

Iwasaki has proposed in IEEE Transactions on Magnetics, Vol. MAG-16, No.1, January 1980, pages 71 to 76 a perpendicular magnetic recordingsystem which theoretically makes it possible to produce a higherrecording density than one produced by using the longitudinalmagnetization mode. In the perpendicular magnetic recording system, themagnetization perpendicular to the surface of the magnetic recordinglayer is used for recording.

The magnetic layer adapted to the perpendicular magnetization systemshould be an alloy layer mainly consisting of cobalt and additionallychromium and should have a magnetic anisotropy perpendicular to thelayer surface. This magnetic anisotropy, i.e., perpendicular magneticanisotropy, should usually have the relationship HK≧4πMs, wherein Hk and4πMs are the anisotropy field and the maximum demagnetizing field of amagnetic layer, respectively. This relationship designates that themagnetic layer possesses a satisfactorily high perpendicular anisotropy.

In the alloy layer mentioned above, the direction of easy magnetization,i.e., the C axis of the hexagonal cobalt alloy, is orientedperpendicular to the layer surface. Such orientation is referred to asperpendicular orientation and is evaluated by subjecting a magnetic filmto X-ray diffraction, obtaining the rocking curve of the diffractionpeak from the (002) plane of the hexagonal closest packing (hcp)structure, and measuring the half value width Δθ₅₀ of the rocking curve.A half-value width Δθ₅₀ of 10° or less is alleged to be sufficient forobtaining excellent perpendicular anisotropy. The coercive force H_(cv)in the perpendicular direction, which is more than 100 Oersted (Oe), isallegedly sufficient for obtaining an excellent perpendicularorientation.

U.S. Pat. No. 4,210,946 proposes a perpendicular magnetic recordingmedium (hereinafter referred to as a two-layer film) which is suitablefor effectively recording and/or regenerating signals from asingle-pole-type magnetic recording head. More specifically, U.S. Pat.No. 4,210,946 discloses a layer of low coercive force materialconsisting of Permalloy and a layer of a 5% to 25% by weight ofchromium-cobalt alloy successively deposited on a nonmagnetic base bymeans of an RF sputtering method, the target electrode and the basebeing disposed opposite to one another. The two-layer film disclosed inU.S. Pat. No. 4,210,946 allegedly provides a high recording density anda high output.

It is known that the half-value width Δθhd 50 of a cobalt alloy, isincreased and the perpendicular magnetic anisotropy, is deterioratedmore when Permalloy is used for the layer of low coercive forcematerial, as compared with the half value width Δθ₅₀ a perpendicularmagnetic recording medium which does not comprise a layer of lowcoercive force material (Uesaka et al. Technical Report S7-1. "Two-LayerFilms for Perpendicular Recording Medium"). This perpendicular magneticrecording medium is hereinafter referred to as a one-layer film. In theRF sputtering method used in U.S. Pat. No. 4,210,946, it is necessary touse as the nonmagnetic base an expensive heat-resistant macromolecularmaterial film, such as a polyimide film, because the temperature of thenonmagnetic base is increased during RF sputtering. If an inexpensivemacromolecular material film, such as a polyester film, is used as thenonmagnetic base, the deposition rate film is decreased and the RFsputtering device must be provided with a specified cooling means. An RFsputtering method cannot be applied in the large-scale production of orhigh-speed growth of perpendicular magnetic recording mediums becausethe highest growth rate of a cobalt alloy layer which can be achieved atpresent by means of the RF sputtering method is about 500 Å per minuteeven when a polyimide film is used as the nonmagnetic base. In addition,since the one-layer film or two-layer film obtained by means of the RFsputtering method exhibits a poor flexibility, it may cause the magnetichead to wear or may be damaged by the magnetic head when used forrecording or regenerating signals.

It is an object of the present invention to provide a two-layer film inwhich the perpendicular orientation is not reduced due to the layer oflow coercive-force material.

It is another object of the present invention to provide a method forproducing a two-layer film at such an enhanced rate of production as tomake the method commercially applicable. The method should make itpossible to use a less expensive and a low heat-resistant film, such asa polyester film, as the nonmagnetic base of the two-layer film.

It is yet another object of the present invention to provide asputtering device which allows uniform and high-peed formation of amagnetic layer, particularly a perpendicular magnetic recording layer,and a layer of low coercive-force material.

In accordance with the objects of the present invention, there isprovided a perpendicular magnetic recording medium (a two-layer film)comprising a nonmagnetic base and two magnetic layers successivelyformed on the nonmagnetic base, i.e., a layer of low coercive-forcematerial and a layer of a cobalt alloy and having a direction of easymagnetization in a direction perpendicular to the film surface,characterized in that the layer of low coervice-force material consistsof an alloy which is mainly composed of cobalt and additionallytantalum.

The two-layer film according to the present invention is characterizedby using as the layer of low coercive-force material an alloy(hereinafter referred to as a Co-Ta alloy which is mainly composed ofcobalt and additionally contains tantalum. This alloy can provide acobalt alloy (hereinafter referred to as a Co-Cr alloy) layer having avery improved perpendicular orientation, which in turn leads to providethe two-layer film having a high recording density. In the perpendicularmagnetic recording technique, to attain a high recording density, it isimportant that the perpendicular coercive force H_(cv) of theperpendicular magnetic recording medium matches the magneticcharacteristics of the recording and/or regenerating head. In accordancewith the characteristics of a magnetic recording system to which theperpendicular magnetic recording medium is applied, the perpendicularcoercive force H_(cv) is determined within the range of from 200 to 1300Oe. In determining the perpendicular coercive force H_(cv), it iscrucial that the magnetic anisotropy of the Co-Cr alloy layer in termsof the half-value width Δθ₅₀ be excellent. The perpendicular magneticanisotropy of the two-layer film of the present invention is veryexcellent and is surprisingly superior to that of a one-layer film. Thereason why such an excellent perpendicular magnetic anisotropy can beobtained is not clear but appears to be as follows.

When there is only a trace of Ta in the Co-Ta alloy layer, the mostenergentically stable Co crystals are those which are orientedperpendicular to the surface of the Co-Ta alloy layer, and in which thedistance between the C planes of hcp crystals are enlarged due to Taatoms. When the Co-Cr alloy is deposited on the surface of the Co-Taalloy layer contain only a trace of Ta, mismatching of the crystallattices occurs locally between the Co-Ta alloy layer and the Co-Cralloy layer because the lattice constants of these two alloys areslightly different from one another. In this case, the perpendicularorientation of the Co-Cr alloy is low.

When the Ta concentration of the Co-Ta alloy layer becomes high, the Coatoms in the Co-Ta alloy is disarranged as compared with that of the hcpcrystals, that is, disordering of the Co atoms takes place. Since theCo-Ta alloy tends to exhibit unclear grain boundaries in proportion tothe degree of disordering of the Co atoms, the Co-Ta alloy layer isuniform when observed microscopically. Such uniformity results in asmooth Co-Ta alloy layer surface as well as in the elimination of localmismatching between the Co-Cr alloy crystals and the Co-Ta crystals.When the Co-Cr alloy is deposited on the uniform and smooth Co-Ta alloylayer, Co atoms of these layers are brought into contact with each otherat the beginning of deposition, and during deposition the Co crystalsgrow perpendicularly and form crystal lattices which are orientedperpendicular to the film surface.

In accordance with the objects of the present invention, there is alsoprovided a method for producing a perpendicular magnetic recordingmedium (two layer film), comprising the steps of: forming said Co-Taalloy layer by a sputtering method (hereinafter referred to as anopposed target sputtering method), wherein a magnetic field is generatedin a direction perpendicular to the surfaces of a pair of targetsarranged opposite to one another within a sputtering device, and saidhcp cobalt alloy layer is deposited on the base, which is located besidea space between said pair of targets and which faces said space; and,forming said cobalt alloy layer by the opposing target sputteringmethod.

In accordance with the objects of the present invention, there is alsoprovided a sputtering device, comprising:

a vacuum vessel;

at least one pair of opposed targets disposed in the vacuum vessel;

a means for generating a magnetic field between the at least one pair ofopposed targets in a direction perpendicular to opposed the targets, themeans being located behind the targets; and

at least one pair of conveyable holders for a nonmagnetic base, each ofthe conveyable holders being located beside and facing a space betweenthe at least one pair of targets, and being conveyed in a directionperpendicular to the targets, layers having the composition of thetargets being deposited on the nonmagnetic base by sputtering.

The embodiments of the present invention are described hereinafter.

According to one embodiment, the Ta concentration of the Co-Ta alloy isat least 15% by weight or at least 6.4 atomic %. In this case, thecoercive force in plane H_(c) of the Co-Ta alloy is very low, e.g., 100Oe at the highest. Furthermore, the Co-Cr alloy layer on the Co-Ta alloylayer has a half-value width Δθ₅₀ of ten degrees or less.

According to another embodiment of the present invention, the Co-Taalloy, i.e., the layer of low coercive-force material alloy, isamorphous. An amorphous Co-Ta alloy exhibits no magnetic anisotropy.That is, an amorphous Co-Ta alloy has no magnetic anisotropy imparted toit by its crystal structure. In addition, an amorphous Co-Ta alloyexhibits a very low coercive force in plane H_(c) of, for example, 5 Oeor less, a very low half-value width Δθ₅₀ of four degrees or less, ahigh permeability, and a high resistivity. The two-layer film, in whichthe Co-Ta layer is amorphous and thus exhibits the above-describedproperties, is very effective for enhancing the recording sensitivitywhen it is used for high-density and high-speed recording. Aconventional layer of low coercive-force material, i.e., crystallinematerial, such as permalloy, has a magnetic anisotropy which results ina reduction in permeability and an increase in watt loss, includinghysteresis loss and eddy-current loss. Therefore, when a conventionaltwo-film layer comprising a Permalloy layer is used for high-densityrecording, the S/N ratio is disadvantageously low. This disadvantage canbe eliminated by using the amorphous Co-Ta alloy layer of the presentinvention.

In addition, since the amorphous Co-Ta alloy of the present inventionexhibits the above-described properties, the layer of low coercive-forcematerial can be made very thin, which is advantageous from an economicalpoint of view. In addition to the above-described properties, the Curiepoint of the amorphous Co-Ta alloy is high. It is therefore possible toattain magnetic characteristics which is thermally stable. Furthermore,the amorphous Co-Ta alloy is highly corrosion-resistant and is thereforeadvantageous for practical use.

According to an embodiment of the present invention, the Coconcentration of the Co-Ta alloy is at least 50 atomic % (24 wt %).Preferably, the Co concentration of the Co-Ta alloy is virtually thesame as that of the Co-Cr alloy. According to this embodiment, theinterface between the Co-Ta alloy layer and the Co-Cr alloy layer ismechanically very stable because both of the layers have expansioncoefficients and specific heats which are commensurate to each other andfurther because the wettability between the layers is good. In addition,a solid solution may form at the interface between the Co-Ta alloy andthe Co-Cr alloy.

A conventional layer of low coercive-force material, e.g., an iron-basedalloy, such as Permalloy, is liable to oxidize or undergo deteriorationof its properties during the formation thereof.

According to another embodiment, the Co-Cr alloy contains from 10% to25% by weight of Cr and may contain an additional element or elements,such as W, Mo, Ru, Pt, Os, Ni, Re, Ta, or the like. The concentration ofthe additional element or elements must be such that the knownperpendicular magnetic anisotropy induced due to C-axis orientation inthe hcp crystals is not impaired. Preferably, Ta is contained in theCo-Cr alloy at a concentration of from 2 to 10 atomic % with the provisothat sum of Ta and Cr concentrations is 27 atomic % at the highest.

According to yet another embodiment, the nonmagnetic base consists of apolyimide film or, preferably, a polyester film.

The preferred embodiments of the present invention are hereinafterdescribed with reference to the drawings, wherein:

FIG. 1 is a sputtering device used for implementing the method of thepresent invention;

FIG. 2 is a target used for forming a Co-Ta alloy layer;

FIG. 4 is a means for holding a nonmagnetic base;

FIGS. 3, 5, and 6 are graphs illustrating the experimental resultsobtained in Example 1;

FIGS. 7 and 8 illustrate embodiments of the sputtering device accordingto the present invention;

FIG. 9 is a partial view of FIG. 7;

FIG. 10 illustrates the arrangement of magnets in a target;

FIGS. 11 through 13 illustrates sputtering devices which can be used forimplementing the method of present invention; and

FIGS. 14 and 15 are drawings illustrating the magnetic flux density anderosion of a target, respectively.

Referring to FIG. 1, a sputtering device with a pair of opposed targetsis illustrated. This sputtering device, with a pair of opposed targets,which is used to prepare films made of perpendicular-oriented materialsis disclosed in European Patent Publication No. 0054269.

The sputtering device with a pair of opposing targets, is hereinaftersimply referred to as opposed targets sputtering device. The devicecomprises a vacuum vessel 10 and a pair of targets T₁, T₂ which areclosely attached or secured to the target holders 15, 16. The targetsT₁, T₂ are arranged opposite one another so that their surfaces, whichare subjected to sputtering, i.e., the sputtering surfaces T_(1S),T_(2S), face one another over the space between the targets T₁, T₂ whichare parallel to one another.

The target holders 15, 16 are secured to the side plates 11, 12 of thevacuum vessel 10 via the insulating members 13, 14. The targets T₁, T₂,as well as the permanent magnets 152, 162, are cooled by water, which isadmitted into the target holders 15, 16 via the cooling conduits 151,161. The permanent magnets 152, 162 are means for generating a magneticfield perpendicular to the sputtering surfaces T_(1S), T_(2S) and arearranged in such a manner that the N pole of one of the permanentmagnets faces the S pole of the other permanent magnet. A magnetic fieldis generated only between the targets T₁, T₂. The target holders 15, 16and the insulating members 13, 14 are protected by the shields 17, 18from plasma particles formed during sputtering. The shields 17, 18prevent an abnormal electric discharge from occuring at places otherthan the targets T₁, T₂.

The nonmagnetic base 40 on which the magnetic layers are formed by theopposed target sputtering method is located on the base holder 41disposed beside the targets T₁, T₂ so that the nonmagnetic base 40 islocated beside the space between the targets T₁, T₂ and faces thisspace. The base holder 41 is usually positioned perpendicular to thesputtering surfaces T_(1S), T_(2S).

Reference numeral 50 denotes a sputtering power source, which is adirect current source to which the targets T₁, T₂ and a ground terminalare connected as a cathode and an anode, respectively. The sputteringpower is applied between the targets T₁, T₂ and the grounded vacuumvessel.

A retractable shutter (not shown) is disposed between the nonmagneticbase 40 and the targets T₁, T₂ as to protect the nonmagnetic base 40from plasma during the pre-sputtering period. The vacuum vessel 10 isprovided with a gas-exhaust port which communicates with a gas-exhaustsystem 20 and a gas-intake port which communicates with a gas source 30.

When operating the opposed-targets sputtering device described above,the gas exhaust system 20 is preliminarily operated so as tosatisfactorily withdraw the gas in the vacuum vessel 10 through the gasexhaust port, and, subsequently, a sputtering gas, such as an argon gas,is admitted into the vacuum vessel 10 from a gas source 30 so that thepressure in the vacuum vessel 10 is increased to a predetermined level,for example, from 10⁻¹ to 10⁻⁴ Torr.

In the opposed targets sputtering device shown in FIG. 1, the magneticfield H is perpendicular to the sputtering surfaces T_(1S), T_(2S). Dueto the layout and configuration of the targets T₁, T₂, high-speedsputtering at a low temperature can be realized. That is, the ionizedsputtering gas and gamma electrons which are expelled from the sputteredtargets are confined in the space between the targets T₁, T₂, with theresult that high-density plasma is formed between the targets T₁, T₂. Itis believed that high-speed growth of the magnetic layers can beachieved by confinement of the high-density plasma. Since thenonmagnetic base is offset from the targets T₁, T₂, heat generation dueto the impinging effects of the electrons on the nonmagnetic base 40 isnot appreciable and therefore magnetic layers can be formed at a lowtemperature.

Referring to FIG. 2, a preferred embodiment of a target is illustrated.The surface of the target is divided into eight fan-shaped zones. Thefan-shaped zones II (Ta) consist of 100% Ta and the fan-shaped zones I(Co) consists of 100% Co. The Ta concentration of the Co-Ta alloy can beadjusted by determining the proportion of the surface area of the formerzones to that of the latter zones.

The opposed-targets sputtering device shown in FIGS. 7 and 8 comprisesthe vacuum vessel 410. The vacuum vessel 410 is provided with agas-exhaust port 440 and a gas-intake port 450 which are connected tothe not-shown gas-exhaust system and to the not shown gas source,respectively.

The opposed-targets sputtering device is provided with a plurality ofpairs of opposed targets which are arranged in rows and which realizemulti-stage sputtering. More specifically, such plurality of pairsconsists of the first pair (T₁) of targets TA₁ and TB₁ and the secondpair (T₂) of targets TA₂ and TB₂. The target holders 411, 412, and 413,are secured to the side walls 410A and 410B (FIG. 8) of the vacuumvessel 410 and are spaced at a predetermined distance therebetween. Thefirst pair (T₁) and second pair (T₂) are therefore arranged in a row.The targets TB₁, TA₂ are secured to a single target holder, i.e., thetarget holder 412. The target holders 411, 412, and 413 are nonmagneticand hollow, and the conduits 411A, 412A, and 413A for water cooling areinserted into the hollow spaces thereof. That is, holding and cooling ofthe targets TA₁, TA₂, TB₁, and TB₂ are achieved by a rather compactmeans.

The permanent magnets are denoted by 442, 443, and 444 and are arrangedso as to generate a magnetic field only between the opposed targets andwhich is directed perpendicular to the surface of the targets. Sinceeach permanent magnet has a cylindrical shape, the magnetic field (notshown) is generated in the form of a cylindrical wall between theopposed targets. Magnetic field-generating means, such as the permanentmagnets 442, 443, 444, are located behind the targets TA₁, TA₂, TB₁, TB₂and the polarities of all of the permanent magnets are preferablyoriented in the same direction, as is shown in FIG. 7. A pair of targetsTB₁ and TA₂, which are arranged in a portion other than the end portionsof the vacuum vessel, is provided with a common magneticfield-generating means, i.e., the permanet magnet 443, and is secured toboth ends of a common target holder, i.e., target holder 412.

A nonmagnetic base-conveying means 470 (FIG. 7) is adaptable forconveying a long, flexible strip of macromolecular material. Morespecifically, the nonmagnetic base-conveying means 470 comprises a reel480, from which the nonmagnetic base 420 (FIG. 7) is uncoiled, androtatable conveying rollers 481U, 482U, 483U, 481D, 482D, and 483D whichdefine a U-shaped conveying pass of the nonmagnetic base 420, and acoiler 490 which coils the nonmagnetic base 420 at a predeterminedspeed. The rotatable tensioning rollers 491U, 492U, 493U, 491D, 492D,and 493D are secured to the side walls 410A and 410B (only the rotatableconveying rollers 491D and 493D are shown in FIG. 8). These rotatabletensioning rollers are arranged so that the nonmagnetic base 420successively passes the upper and lower ends of the spaces S₁ and S₂between the opposed targets T_(A1), T_(A2), T_(B1), and T_(B2). Thenonmagnetic base-supporting plates 421U, 422U, 421D, and 422D arearranged beside the above-mentioned upper and lower sides of the spacesS₁ +S₂, and when the nonmagnetic base 420 slides on the plates, it maybe heated or cooled by heating or cooling equipment (not shown)installed behind the plates. The heating means may be an electric heateror a heating-medium circulating means. The cooling means may be acooling-medium circulating means. Rotatable tensioning rollers 491U,492U, 493U, 494U, 491D, 492D, 493D, and 494D are arranged in front ofand behind the nonmagnetic base-supporting plates 421U, 422U, 421D, and422D so as to bring the nonmagnetic base 420 into a tight contact withthe supporting plates when the nonmagnetic base 420 is being conveyed.Shields are denoted by 446, 447, and 448 and surround the target holders411, 412, and 413. The opposed-targets sputtering device is providedwith the power sources 459, and 461.

The first and second pairs T₁ and T₂ of targets may comprise targetshaving the same composition. In this case, the deposition rate of amagnetic film can be four times as high as that attained by theopposed-targets sputtering device shown in FIG. 1.

The nonmagnetic base 420 is subjected to the deposition of a magneticfilm on both the upper and the lower ends of the spaces S₁ and S₂. As aresult, the rate deposition of the magnetic film per one pair of opposedtargets is twice as high as that attained in the opposed targetssputtering device shown in FIG. 1.

Referring to FIG. 9, the nonmagnetic base-supporting plate 421D isillustrated. The nonmagnetic base-supporting plate 421D is electricallyinsulated from the vacuum vessel (not shown) and is electricallyconnected to the power sources B₁, B₂ and B₃. The nonmagneticbase-supporting plate 421D is separated by the electrically insulatingbodies I into three electrode sections E₁, E₂, and E₃, which areconnected to the power source B₁, B₂, and B₃, respectively. A biaspotential determined by each of the power source B₁, B₂, and B₃ isapplied to each of the electrode sections E₁, E₂, and E₃. A negativebias potential decreases the impact energy of the gamma electrons andthe like when they are deposited on the nonmagnetic base 420. On theother hand, a positive bias potential increases the impact energy of thegamma electrons and the like when they are deposited on the nonmagneticbase 420. Since the kinetic energy of the gamma electrons and the likeis not uniform within the space between the targets T.sub. A1 andT_(B1), the rate deposition of the magnetic film on the nonmagnetic base420 tends to be nonuniform. The nonmagnetic base-supporting plate 421Dshown in FIG. 9 is advantageous for forming a magnetic film which has asensitive crystal structure, such as a Co-Cr alloy film.

Referring to FIG. 10, individual permanent magnets 445a and 445b arearranged in the target holder 412. Therefore, each target is providedwith one magnetic field-generating means.

It is preferred in the opposed targets sputtering device according tothe present invention that the magnetic field-generating means comprise:a first means for generating a magnetic field around a pair of targets,said first means having such a configuration as to surround the pair oftargets; and a second means for producing a magnetic flux, said secondmeans being connected to said first means via a magnetic path formedbetween the first and second means.

Referring to FIG. 11, the opposed-targets sputtering device is providedwith a pair of targets T₁ and T₂, target holders 311 and 312, andconduits 311a and 312a. The vacuum vessel is denoted by 310. The firstmeans comprise cores 301 and 302, which are electrically connected tothe vacuum vessel 310. The cores 301 and 302 have the same configurationas the shields 17, 18 in FIG. 1 and may be cylindrical. The insulatingspacers 315 and 316 are inserted between the cores 301 and 302 and thetarget holders 311 and 312, so that a distance of a few millimeters iscreated. The cores 301 and 302 are provided at the top ends thereof withfront portions 301a and 302a, which are opposed and between which amagnetic field is generated. The cores 301 and 302 and their frontportions 301a and 302a may be made of mild steel, silicon steel,Permalloy, or other soft magnetic materials having a high permeabilityand a high saturation magnetization. The second means may be amagnetizing coil or a permanent magnet. In FIG. 11, the second means aretwo magnetizing coils 301' and 302' which are located outside the vacuumvessel 310. When the magnetizing coils 301' and 302' mounted on thecores 301 and 302 are energized, the cores 301 and 302 produce amagnetic field H. The intensity of the magnetic field H can be easilyadjusted by controlling the current of the magnetizing coils 301' and302'.

It is preferred in the opposed-targets sputtering device according tothe present invention that one end of a magnetic-field generating meanssaid end being closest to the targets, consist of soft magnetic materialhaving a high permability. Referring to FIG. 10, for example, thepermanent magnet 445a comprises a magnet body 445a' and a tip 446a whichconsists of soft magnetic material having a high permeability and a highsaturation magnetization. Since the demagnetizing field induced in thepermanent magnet 445a, can be decreased by the tip 446a, the magneticflux is concentrated around the outer periphery of the target T_(B1).Erosion of targets TA₂ and TB₁ can be uniformly eroded; but this is notthe case when the cylindrical permanent magnet 445b is used. It ispreferred that the tip have a pointed configuration at its outside frontend.

Referring to FIGS. 14, and 15, it is illustrated how the erosion totargets made of Co-20 wt. % Cr alloy is varied by changing theconstruction of the magnetic field generating means. Symbols used inthese figures indicate the following.

TG₁ : The permanent magnets were as shown in FIG. 1 and the sputteringpower was 1045 w.

TG₂ : The permanent magnets were as shown in FIG. 10 were used and thesputtering power was 1027 w.

TG₃ : The magnetic field generating means as shown in FIG. 11 was usedand the sputtering power was 1079 w.

It will be apparent that in TG₃ erosion of the target is the mostuniform Distribution of erosion and magnetic flux over the target in TG₂are very uniform as compared with those of TG₁.

The method for producing the two-layer film according to the presentinvention may be carried out by using not only the opposed-targetssputtering devices shown in FIGS. 1, 7, 8, and 11 but also by using theopposed-targets sputtering devices shown in FIGS. 12 and 13. In FIG. 12,the same members as those in FIG. 1 are denoted by the same referencenumerals. The magnetic-field generating means in FIG. 12 is amagnetizing coil 430 disposed outside the vacuum vessel 10. The opposedtargets sputtering device shown in FIG. 13 is provided with the firstand second means described with reference to FIG. 11 and thenonmagnetic-base conveying means described with reference to FIGS. 7 and8, as will be apparent from the reference numerals given in thesedrawings. It should be understood that the opposed-targets sputteringdevices shown in FIGS. 1, 7, 8, 11, 12, and 13 are not limitative at allfor carrying out the method of the present invention.

In carrying out the method of the present invention, it is preferredthat the Co-Cr alloy layer be formed on the Co-Ta alloy layer no laterthan ten hours after formation of the Co-Ta alloy layer. The Co-Cr alloylayer is highly likely to peel off of the Co-Ta alloy layer if thenonmagnetic base having Co-Ta alloy layer thereon is cooled in vacuum toroom temperature, is taken out of the opposed target sputtering device,is exposed to ambient air for a long period of time, and is subsequentlysubjected to the formation of Co-Cr alloy layer. The surface, of thesealloy layers are very smooth. If the exposure time of the Co-Ta alloylayer to the ambient air is less than ten hours, the adhesion of theCo-Ta alloy layer and the Co-Cr alloy layer is acceptable practically.If the Co-Ta alloy layer is not at all exposed to the ambient air andthe Co-Cr alloy layer is immediately formed on the Co-Ta alloy layer,the adhesion is excellent.

The present invention is now explained by way of examples.

EXAMPLE 1

Samples of the perpendicular magnetic recording medium were preparedunder the following conditions:

A. The opposed-Targets Sputtering Device (FIG. 1)

(1) Material of the Targets T₁ T₂ : Target T₁ (100 atomic % Co), TargetT₂ (100 atomic % Co - I (Co) - and 100 atomic % Ta - II (Co) -.

(2) Distance Between the Targets T₁, T₂ : 75 mm

(3) Magnetic Field in the Neighborhood of the Targets T₁, T₂ : 100˜200gausses

(4) Dimension of the Targets T₁, T₂ : 110 mm in diameter (Round DiscTargets)

(5) Distance of the Nonmagnetic Base 40 From the Ends of the Targets T₁,T₂ : 30 mm

B. Nonmagnetic Base 40:

A 25 μm thick Capton film (Aromatic polyimide film produced by Dupont bythe tradename of Capton) and a 16 μm thick polyethylene terephthalate(PET) film (Both of these films were used in the experiments) wereproduced.

The two-layer fims were produced by the following procedure.

The nonmagnetic base 40 was first fixed on the base holder 41 and thenthe gas in the vacuum vessel 10 was exhaused until an ultimate degree ofvacuum of 1×10⁻⁶ Torr or less was achieved. Subsequently, an argon gaswas admitted into the vacuum vessel 10 until the pressure was increasedto 4 mm Torr. After pre-sputtering for three to five minutes, theshutter (not shown in FIG. 1) was retracted and the formation of a Co-Taalloy layer on the nonmagnetic base 40 was initiated. The electric powerduring sputtering was 250 w or 500 w and a 0.55 μm thick Co-Ta alloylayer was formed. This procedure was repeated while varying the Taconcentration of the Co-Ta alloy layers. The coercive force in planeH_(c) and the saturation magnetization M_(s) of the produced Co-Ta alloylayers were measured. The results are shown in FIG. 3.

As will be apparent from FIG. 3, the coercive force in plane H_(c) was100 Oe or more and the saturation magnetization was 1100 emu/cc or morewhen the Ta concentration of the Co-Ta alloy was 15% by weight (5.4atomic %) or less. The coervice force in plane H_(c) and the saturationmagnetization M_(s) decreased with an increase in the Ta concentration.The Co-Ta alloy had an excellent soft magnetic property, i.e., acoercive force in plane H_(c) of 5 Oe or less, when the Ta concentrationwas 23% by weight (8.9 atomic %) or more.

The Co-Ta alloy layers were subjected to X-ray diffraction analysis.When the Ta concentration was 22% by weight (8.4 atomic %) or less, thediffraction peak was at an angle (2θ) of from 43.80 to 44.02. When theTa concentration was 23% by weight or more, a diffraction peak was notdetected, thus revealing the Co-Ta alloy layer to be amorphous.

Measurement of the resistivity also revealed the Co-Ta alloy layercontaining 23% by weight or more of Ta to be amorphous.

The properties of several samples are given in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Sample         Ta Concentration                                                                            Diffraction                                      No.    Base    wt %          Peak    H.sub.c (Oe)                             ______________________________________                                        I - 1  Capton   9.2          Detected                                                                              121                                      I - 2  PET     22.5          Detected                                                                              9.3                                      I - 3  Capton  23.5          None    2.0                                      I - 4  PET     24.0          None    0.5                                      ______________________________________                                    

EXAMPLE 2

The nonmagnetic bases on which a Co-Ta alloy layer was formed accordingto the procedure of Example 1 were cooled in a vacuum down to roomtemperature and then were removed from the opposed-targets sputteringdevice. In each experiment, three of the nonmagnetic bases 40 weremounted on the base holder 41 (FIG. 4) and the Co-Cr alloy layer wasformed on the Co-Ta alloy layer. The Co-Cr alloy layer was formed underthe following conditions:

A. The Opposed-Targets Sputtering Device (FIG. 1)

(1) Material of the Targets T₁, T₂ : Co-Cr alloy containing 17% byweight of Cr

(2) Distance Between the Targets T₁, T₂ : 100 mm

(3) Magnetic Field in the Neighborhood of the Targets T₁, T₂ : 100˜200gauss

(4) Dimension of the Targets T₁, T₂ : 150 mm×100 mm×10 mm (thickness)

(5) Distance of the Nonmagnetic Base 40 From the Ends of th Targets T₁,T₂ : 50 mm

(6) Target holder (FIG. 4)-three nonmagnetic bases 40 were mounted onholders 42 which were secured on a holding body 44 which was rotatedaround a rotatable driving shaft 43. The rotatable driving shaft 43 wererotated at an almost constant speed.

B. Nonmagnetic Base 40

A 25 μm thick Capton film and a 16 μm thick polyethylene terephthalate(DET) film. The Co-Ta alloy layer was formed on these films by the sameprocedure as that used in of Example 1, except that the target holder 40was rotated at 40 rpm and the sputtering power was 1000 w. For thepurpose of comparison, one-layer films were produced by the proceduredescribed above.

The half-value width Δθ₅₀ of the Co-Cr alloy layer of the two-layerfilms and the one-layer films was measured. The results are shown inFIG. 5. As is apparent from FIG. 5, the half-value width Δθ₅₀ oftwo-layer films is very excellent when the Ta concentration (3 degree)is 23% by weight or more. A Ta concentration of 23% by weightcorresponds to the structural change of the Co-Ta alloy, i.e., thecrystal structure is changed to an amorphous structure and vice versa.Surprisingly, the half-value width Δθ₅₀ of the two-layer films was verylow, e.g. 5 degree, and could be decreased more than that of thesingle-layer films, when the Co-Ta alloy layer of the two-layer filmshad an amorphous structure.

The properties of several samples are given in Table 2.

                                      TABLE 2                                     __________________________________________________________________________                                   Co--Cr Alloy Layer                                                 Co--Ta Alloy Layer                                                                             Diffraction                              Sample                                                                            Nonmagnetic                                                                          Ta Concentration                                                                       Diffraction                                                                              Diffraction                                                                         Peak                                     No. Base   (wt %)   Peak  Remarks                                                                            Peak  Δθ.sub.50                    __________________________________________________________________________    II-1                                                                              Capton --       --    Single                                                                              44.51°                                                                       3.5°                             II-2                                                                              "       9.2     43.94 No. I-1                                                                            None  --                                       II-3                                                                              "      15.8     43.85 --   44.53 8.7                                      II-4                                                                              "      23.5     None  No. I-3                                                                            44.55 3.0                                      II-5                                                                              "      23.6     None  --   44.54 2.6                                      II-6                                                                              PET    --       --    Single                                                                             44.57 4.2                                      II-7                                                                              "      22.5     None  No. I-2                                                                            44.60 3.7                                      II-8                                                                              "      38.8     None  --   44.58 3.6                                      __________________________________________________________________________     Remarks                                                                       (1) The diffraction Peak of a (002) plane was measured and is given by        angle (2θ).                                                             (2) "Single" indicates a onelayer film.                                  

As is apparent from Table 2, the Co-Ta alloy is crystalline when theTa-concentration is 22% by weight (8.4 atomic %) or less. The crystalstructure of the Co-Ta alloy is an hcp structure and the Co-Ta crystalsare oriented along the C-axis of the hcp structure. The distance betweenthe C planes of the Co-Ta crystals is greater than that of the Cocrystals.

The surface and cross section patterns of two-layer films wasinvestigated by means of a diffraction electron microscope produced byJapan Electron Co., Ltd. (JSM-35C type).

The specimens for observing the surface pattern were prepared bydepositing an Au-Pd layer on the perpendicular magnetic recording layersto a thickness of approximately 200 Å. Electron microscopic photographswere taken at a magnification of 40,000 and under an accelerationvoltage of 25 kV. The specimens for observing the cross section patternwere prepared by putting the two layer films into a gelation capsuletogether with ethyl alcohol, cooling the capsule with liquid nitrogenfor two hours, and then cleaving the capsule with a cleaving knife. Thedevice used for the freeze-cleaving method was a TF-1-type deviceproduced by Eiko Engineering Co., Ltd.

The surface pattern of the Co-Cr alloy layer was composed of uniformparticles of 500 Å or less in size and the cross section patterns of theCo-Cr alloy layer and the Co-Ta alloy layer exhibited virtually no grainboundaries and were composed of a few fragmented particles which weredispersed. A very flat boundary was observed between the Co-Cr alloylayer and the Co-Ta alloy layer. The adhesion of these layers to eachother was tested by changing the time between the completion offormation of the Co-Ta alloy layer and the initiation of formation ofthe Co-Cr alloy layer. When the Co-Ta alloy layer was exposed to theambient air for a few days, the above-mentioned adhesion was very poorand the Co-Cr alloy layer was very susceptible to peeling. Asatisfactorily high adhesion could be obtained by keeping the exposuretime shorter than ten hours.

EXAMPLE 3

The procedure of Example 2 was repeated except for the following:

A. Distance of the Nonmagnetic Base 40 From the Ends of the Targets: 25mm

B. Target Holders:

The holders 42 were provided with a cooling means (not shown in FIG. 4)located behind them.

C. The nonmagnetic bases 40 were kept stationary during sputtering. Theelectric power amounted to 5 kW at the highest during sputtering and thethickness of the Co-Cr alloy layer was approximately 0.5 μm. Thedeposition rate was varied in the range of from approximately 0.1 μm/minto approximately 0.7 μm/min.

The relationship between the half-value width Δθ₅₀ and the depositionrate (Rd) is shown in FIG. 6. In FIG. 6, the symbols ○ and indicateone-layer films and two-layer films, respectively, in which a PET filmwas used as the nonmagnetic base 40. As is apparent from FIG. 6, it ispossible to produce two-layer films having an excellent half-value widthΔθ₅₀ at a high deposition rate of up to approximately 0.7 μm/min byusing a PET film as the nonmagnetic base.

In the present example, the influence of cooling upon the properties oftwo-layer films was tested. In the test, the holders 42 (FIG. 4) weremade of a stainless steel sheet having a surface roughness of from 0.1Sto 0.6S and the nonmagnetic bases 40 were cooled via the holders 42,behind which a cooling chamber (not shown) was defined. The temperatureof the holders 42 was varied in the range of from 25° C. to 80° C., andthe vertical coercive force H_(cv) of the Co-Cr alloy layer was variedfrom 200 to 400 Oe.

When the holders 42 made of a mat- or satin-finished stainless steelsheet was used in the test mentioned above the temperature of theholders was 25° C., vertical coercive force H_(cv) was 830 Oe. In thiscase, the deposition rate was 0.3 μm/min.

In the case of both the holders made of a 0.1S-0.6S stainless steelsheet and the holders made of a satin-finished stainless steel sheet,the half-value width Δθ₅₀ was approximately 3 degrees.

EXAMPLE 4

A conventional two-layer film and a two-layer film according to thepresent invention were prepared according to the procedure of Example 2.

The layer of low coercive-force material of the conventional two-layerfilm consisted of an alloy comprised of 4% Mo, 78% by weight of Ni, and18% by weight of Fe. The Co-Ta alloy layer of the two-layer filmaccording to the present invention contained 30% by weight of Ta. Theproperties of these films are given in Table 3.

                  TABLE 3                                                         ______________________________________                                        Layer of Low                                                                  Coercive-Force   Co--Cr Alloy Layer                                           Material                       Half-                                                         Coercive        Vertical                                                                              Value                                         Thick-  Force     Thick-                                                                              Coercive                                                                              Width                                  Sample ness    in plane  ness  Force (H.sub.cv)                                                                      Δθ.sub.50                  No.    (μm) (Oe)      (μm)                                                                             (Oe)    (degree)                               ______________________________________                                        IV - 1                                                                              0.5      1.0       0.5   230     3.0                                    IV - 2                                                                              0.4      1.0       0.5   360     10                                     ______________________________________                                    

The recording characteristic of the above-mentioned two films wasmeasured by using the perpendicular magnetic head described in IEEETrans. on Mag., Vol. MAG-16, No. 1, Jan. 1980, page 71.

The regenerating peak-to-peak voltage was measured while the kilo fluxreversal per inch (KFRPI) was varied from 1.0 to 100. The results aregiven in Table 4.

                  TABLE 4                                                         ______________________________________                                               Recording Condition (KFRPI)                                                     1.0    2.0     5.0  10    20   50    100                             Sample No.                                                                             mV     mV      mV   mV    mV   mV    mV                              ______________________________________                                        IV - 1   160    160     160  150   150  140   80                              (Invention)                                                                   IV - 2   240    240     230  210   160  90    25                              (Prior art)                                                                   ______________________________________                                    

As is apparent from Table 4, when the recording condition in terms ofKFRPI was from 1 to 5, the regenerating peak-to-peak voltage of SampleNo. IV-1 was higher than that of Sample No. IV-2. This was due to thefact that the vertical coercive force H_(cv) of Sample No. IV-2 washigher than that of Sample No. IV-1. However, the peak-to-peakregenerating voltage of Sample No. IV-2 drastically decreased when theKFRPI was 50 or more. Sample No. IV-1 did not exhibit such a drasticdecrease at all, and it is believed that the reason for this was anexcellent half-value width Δθ₅₀.

EXAMPLE 5

The nonmagnetic bases on which a Co-Ta alloy layer was formed accordingto the procedure of Example 1 (Sample No. I-4) were cooled in a vacuumdown to room temperature and then were removed from the opposed-targetssputtering device.

The procedure of Example 1 was repeated for forming a Co-Cr alloy layerexcept that the following was changed.

(1) Target T₁ : A Ta plate consisting of 100% of Ta was positioned on aportion of the Co-Cr alloy plate containing 17 atomic % of Cr

(2) Target T₂ : A plate consisting of Co-Cr alloy containing 17 atomic %of Cr

(3) Distance between Targets T₁, T₂ : 120 mm

(4) Dimension of Targets T₁, T₂ : 150 mm×100 mm×10 mm (Thickness)

(5) Distance of Nonmagnetic Base 40 From the Ends of Targets T₁, T₂ : 50mm

(6) Distance of the Nonmagnetic Base 40 From the Ends of the Targets T₁,T₂ : 50 mm

During sputtering the electric power was 1000 w and the argon gaspressure in the vacuum vessel was 4×10⁻³ Torr. As a result ofsputtering, a 0.5 μm thick Co-Cr alloy layer which contain Ta wasformed. The Ta concentration of Co-Cr alloy layer was varied by changingthe size of the Ta plate. For the purpose of comparison, a onelayer-film was produced and 16 μm thick PET film by the proceduredescribed above.

The two-layer films and the one-layer film produced were subjected tomeasurement of the perpendicular coercive force Hcv, the coercive forcein plane Hc, the perpendicular residual magnetization, Mrv, the residualmagnetization in plane Mrh, and the anisotropic magnetic field Hk. Theresults are shown in Table 5.

                                      TABLE 5                                     __________________________________________________________________________               Properties of Co--Cr Layer                                                    Composition                                                        Sample                                                                            Nonmagnetic                                                                          (atomic %)  Hcv                                                                              Mrv                                                                              Hcv                                                                              Hk                                            No. Base   Co Cr Ta Δθ.sub.50                                                            Hch                                                                              Mrh                                                                              (Oe)                                                                             (KOe) Remarks                                 __________________________________________________________________________    V-1 PET    80.9                                                                             16.3                                                                             2.8                                                                              4.8                                                                              0.61                                                                             0.18                                                                              62                                                                              1.6                                           V-2 "      79.9                                                                             16.3                                                                             3.8                                                                              3.6                                                                              3.08                                                                             1.18                                                                             222                                                                              2.5                                           V-3 "      79.4                                                                             16.5                                                                             4.1                                                                              4.3                                                                              7.41                                                                             2.54                                                                             335                                                                              2.2                                           V-4 "      77.6                                                                             16.6                                                                             5.8                                                                              4.4                                                                              5.96                                                                             2.65                                                                             402      Two Layer                               V-5 "      77.9                                                                             16.0                                                                             6.1                                                                              5.6                                                                              2.06                                                                             0.92                                                                             175                                                                              1.7   Films                                   V-6 "      77.3                                                                             15.6                                                                             7.1                                                                              4.5                                                                              1.45                                                                             0.81                                                                             123                                                                              2.2                                           V-7 "      77.0                                                                             15.5                                                                             8.5                                                                              4.7                                                                              1.50                                                                             0.82                                                                              65                                                                              1.2                                           V-8 "      83 17 0  3.5                                                                              0.81                                                                             0.25                                                                             145                                                                              2.4   One-Layer                                                                     Film                                    __________________________________________________________________________

As is apparent from Table 5, the half value width Δθ₅₀ of the Co-Cralloy layer containing Ta is slightly inferior to but is virtually thesame as that of the Co-Cr alloy layer of the one-layer film. The halfvalue width Δθ₅₀ of the Co-Cr alloy layer containing Ta is notdeteriorated greatly due to the layer of low coercive force material.

As is apparent from Hcv/Hc and Mrv/Mrh given in Table 5, theperpendicular orientation of the two-layer films is considerablyimproved over that of the one-layer film, when the Ta concentration ofCo-Cr alloy is from 3.8 atomic % to 7.1 atomic %. This makes it possibleto enhance the recording density and recording sensitivity as comparedwith those of a known one-layer film.

We claim:
 1. A sputtering device comprising: a vacuum vessel; at leastone pair of rectangular targets arranged in the vacuum vessel so thattheir long sides are opposed; at least one means for generating amagnetic field along the perimeter of each of said rectangular targetsin a direction perpendicular to the targets, each of said magnetic-fieldgenerating means having an end portion thereof positioned close to oneof said targets, each end portion constituted by a tip made of highpermeability soft magnetic materials, said tip having an outside frontand being shaped into a point at the outside front end thereof; and abase-conveying means for conveying a nonmagnetic base arranged adjacentto the long sides of said at least one pair of said rectangular targetsso that the nonmagnetic base faces a space between said at least onepair of the targets in a direction perpendicular to the long side ofsaid targets, and so that layers having the same composition of saidtargets are deposited on said nonmagnetic base by sputtering.
 2. Asputtering device comprising: a vacuum vessel; at least one pair ofrectangular targets arranged in the vacuum vessel so that their longsides are opposed; at least one means for generating a magnetic fieldoutside and along the perimeter of each of said at least one pair ofrectangular targets in a direction perpendicular to the targets, each ofsaid magnetic-field generating means comprising first means for guidinga magnetic flux around each of said targets with a configuration suchthat each of said targets is surrounded thereby, and second means forproducing a magnetic field and connected to said first means via amagnetic path formed therebetween; and base-conveying means forconveying a nonmagnetic base arranged adjacent to the long sides of saidat least one pair of said rectangular targets so that the nonmagneticbase faces a space between said at least one pair of the targets in adirection perpendicular to the long sides of said targets, and so thatlayers having the same composition of said targets are deposited on saidnonmagnetic base by sputtering.
 3. A sputtering device according toclaim 2, wherein said second means is a magnetizing coil.
 4. Asputtering device according to claim 2, wherein said second means is apermanent magnet.